The invention pertains generally to variable frequency oscillators and is more specifically directed to digitally controlling an output frequency of such oscillators.
Generally, circuits termed as variable frequency oscillators (VFO) have found wide use in the electronics art. These oscillators are generally characterized by operation in the high frequency band to provide a carrier frequency. An input parameter is used to vary the carrier several KHz on either side of its center frequency with the variations being generally linearly related to the input parameter.
Primarily, the utility of such devices is found in angular modulators of both the frequency and phase configurations and their corresponding demodulators. The growing use of the phase locked loop in many applications will however increase their use in the future as the VFO forms an important component of such circuits.
One particularly advantageous method of generating a variable frequency output is by quadrature synthesis. In this method an input information signal and its quadrature are combined with a carrier and its quadrature according to a trignometric identity to form the desired output frequency. For example, a time varying output signal for a variable frequency oscillator may be represented as: EQU V(t)=A cos (wt .+-..phi.) (1)
If A and .phi. are constants, then this equation describes a singular frequency sinusoid of angular frequency w. However, the frequency of the sinusoid may also change according to .phi.(t) because EQU Wi=d.phi./dt (2)
where Wi is the instantaneous change in frequency due to the time change of the phase angle .phi.. Thus the total angular frequency of the sinusoid is: EQU Wt=W+ Wi (3)
Therefore by controlling the phase angle .phi. it is possible to control the output frequency of an oscillator. An identical expression for equation (1) from manipulation with trigonometric identities becomes: EQU V(t)=cos .phi. cos (wt).+-.sin .phi. sin (wt) (4)
Therefore from equations 3 and 4 a variable frequency can be formed by combining the quadrature components of a control signal .phi. (t) with the quadrature components of a carrier of angular frequency W.
U.S. Pat. No. 2,431,569 entitled "Frequency Modulation," issued to E. Labin, and U.S. Pat. No. 3,225,316 entitled "Phase-Shift Single Side-Band Modulators," issued to W. Saraga, illustrate angular modulators using quadrature combinational circuitry.
While the quadrature syntheses for a variable frequency oscillator as described above would be advantageous in many respects, there is some difficulty in the generation of the quadrature phases of .phi. (t), especially if the control signal is broadbanded. The multiplicity of differing frequencies that form a broadband signal do not lend themselves to a simple conversion into quadrature components by a single passive phase shift network.
As a consequence, if the synthesis is to be used, complicated phasing networks such as Hilbert transformers are necessitated. An example of a circuit of the Hilbert type is illustrated in U.S. Pat. No. 3,800,131 entitled "Hilbert Transformer," issued to S. A. White. Other examples of wideband quadrature phasing networks are found in Saraga and Labin, infra. A further difficulty that is encountered in previous quadrature circuits is the omission of the synthesis of the carrier frequency. The carrier frequency and substantially all the synthesized frequencies that fall above or below the carrier frequency are extremely important in many applications. A normal VFO providing phase locking in a phase locked loop would be expected to be able to generate the continuum of frequencies that one might expect to encounter.